Ambient Occlusion Bake Too Dark On Some Meshes on Blender 2.8
The normals are inverted for those portions of the mesh so it's trying to light it inside out. You just need to view the problem parts, select linked and then flip normals.
The new Blender 2.80 quickest way to indicate which way the normals are pointing is in the Viewport Overlays popover (top right of viewport) - toggle option that says 'Face Orientation'.